NATIONAL GRID COMPANY PLC
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
National Grid Company plc 10 September 2003
NATIONAL GRID COMPANY PLC
(A wholly owned subsidiary of National Grid Transco plc)
Investigation Report into the Loss of Supply Incident affecting parts of South
London at 18:20 on Thursday, 28 August 2003
This report has been produced by National Grid Company plc (National Grid) to
record the investigation findings concerning the loss of supply in south London on 28
August 2003. The purpose of the investigation is to enable National Grid to identify
the cause or causes of the incident so it may seek to prevent a recurrence. The
purpose of the report is not, however, to identify legal liability; therefore the data and
information within it have not been compiled in accordance with rules of evidence
and cannot be treated as determining either National Grid’s nor any individual’s legal
liability.
Page 1
National Grid Company plc 10 September 2003
Index page
Executive summary 3
Introduction
Transmission System in South London
Maintenance Activity in the Area
South London Transmission System (diagram)
The First Fault 4
The Second Fault
Restoration 5
Communication
Investigation 6-7
Actions being pursued 7
Investigation Report 9
Introduction
Overview of the incident
Background 10
Transmission system in south London
(Map of affected substations)
Investment programme in the area 11
(London area investment –graph)
Key policies and procedures relevant to the incident 12
Findings
Operating arrangements on the day 13-14
(Table of circuits out of service on 28 August)
Sequence of Events 14-18
Communication during the incident 18
Response to the Buchholz alarm 19
Disconnection of the transformer 20
Unexpected operation of the protection 20-21
Maintenance of assets 21
Other factors 22
Configuration of EDF Energy’s substation 22-23
Communications 23
Investigation 24-25
Actions being pursued 26
Appendices 27-43
Page 2National Grid Company plc 10 September 2003
Investigation Report into the Loss of Supply Incident affecting parts of South
London at 18:20 on Thursday, 28 August 2003.
Executive Summary
Introduction
1 A combination of events led to an electricity power supply failure in south London that
occurred at 18.20 on 28 August. Restoration began at 18.26 and power supplies
from National Grid were fully restored at 18.57. This report describes the
circumstances leading to the loss of supply, the steps taken to restore supplies and
the measures in hand to minimise the risk of a recurrence.
Transmission System in South London
2 The transmission system in south London consists of four substations at Littlebrook,
Hurst, New Cross and Wimbledon. Normal demands of around 1,100MW are drawn
by EDF Energy to supply domestic customers and London Underground, together
with supplies for other large users including NetworkRail. Following the incident
supplies were lost from Hurst, New Cross and part of Wimbledon.
Maintenance Activity in the Area
3 On 28 August 2003, scheduled maintenance was underway on one circuit from
Wimbledon to New Cross and one from Littlebrook to Hurst. This level of
maintenance is usual during the summer months, when demand for electricity is
generally lower.
4 In line with normal practice, the arrangement of the transmission system to
accommodate the maintenance had been agreed with the operator of the distribution
system for the London region, EDF Energy, well in advance, during July 2002.
Routine weekly communication between EDF Energy and National Grid resulted in
the planned outage at Wimbledon proceeding on 1 July 2003. EDF Energy confirmed
that it could arrange its distribution system to accommodate this outage securely for
the maintenance period.
Figure 1: Schematic of the south London transmission system
Northfleet
Willesden 1 1 1 West
Beddington Wimbledon New Cross Hurst Littlebrook
Beddington 2 2 2 Beddington
& Kemsley
Out of service for
scheduled maintenance
Page 3National Grid Company plc 10 September 2003
The First Fault
5 The sequence of events started at 18:11. Engineers at the Electricity National
Control Centre (National Control) received an alarm indicating that a transformer, or
its associated shunt reactor, at Hurst substation was in distress and could fail,
potentially with significant safety and environmental impacts. This “Buchholz alarm”,
told National Control that gas had accumulated within the oil inside the equipment,
which can lead to a major failure. National Grid has approximately 1,000
transformers with associated equipment connected to its transmission system and on
average only 13 Buchholz alarms are received each year.
6 National Control contacted EDF Energy to discuss the Buchholz alarm and asked
EDF Energy to disconnect the distribution system from the transformer. Then, as is
normal practice in this situation, National Control initiated a switching sequence to
disconnect the transformer from the transmission system. This switching sequence
temporarily left supplies dependent on a single transmission circuit from Wimbledon
that feeds New Cross and Hurst substations. Under National Grid operating
procedures a Buchholz alarm is sufficiently serious to warrant the isolation of
equipment and reduced security is acceptable for “switching time”. This is a period of
time, normally around five to ten minutes, during which the transmission system is
rearranged, by connecting and disconnecting circuits, so that the affected equipment
can be taken out of service.
7 The switching sequence to remove the transformer began at 18:20, disconnecting
Hurst substation from Littlebrook substation. This enabled a safe shutdown of the
transformer which had suffered the alarm, but left Hurst supplied only from
Wimbledon via New Cross.
The Second Fault
8 Unexpectedly, a few seconds after the switching, the automatic protection equipment
on the number two circuit from Wimbledon to New Cross operated, interpreting the
change of power flows, due to the switching, as a fault.
9 The transmission system is extensively fitted with many levels of automatic protection
equipment, aimed at isolating faults and preventing damage to equipment or even a
complete shutdown of the transmission system. They measure system
characteristics, such as voltage and current and, in the event of a fault, will
automatically disconnect affected equipment. On the National Grid transmission
system there are approximately 43,000 such pieces of equipment, each with its
individual settings to meet local requirements.
10 The automatic protection relay disconnected the circuit from Wimbledon to New
Cross. This disconnected New Cross, Hurst and part of Wimbledon from the rest of
the transmission system, causing the loss of supply. 724MW of supplies were lost,
amounting to around 20% of total London supplies at that time. This affected around
410,000 of EDF Energy’s customers, with supplies being lost to parts of London
Underground and NetworkRail.
Page 4National Grid Company plc 10 September 2003
Restoration
11 Restoration actions began at 18:26, re-energising the Hurst substation from
Littlebrook and then isolating the Wimbledon to New Cross circuit, that had
automatically disconnected itself, to prevent a recurrence.
12 At 18:38 National Control offered to restore supplies to Wimbledon for EDF Energy.
EDF Energy requested restoration of that supply at 18:48 and restoration was
completed at 18:51. From this point onwards, London Underground could restore
electricity to the underground network, when they considered it was safe to do so.
13 At 18:41 EDF Energy restored supplies via National Grid’s Hurst substation to
approximately one third of the consumers.
14 Some 30 switching actions enabled National Grid to restore overall supplies to all
substations concluding with New Cross at 18:57 which restored the remaining
supplies for NetworkRail. The substations remained connected to the rest of the
transmission system via a single circuit until 23:00, the time at which the automatic
protection equipment that had operated at Wimbledon was successfully isolated.
The number two circuit from Wimbledon to New Cross was then safely returned to
service and normal levels of security were restored. A rapid check was made to
similar automatic protection equipment.
Communication
15 During the incident there was significant operational communication between
National Grid and EDF Energy. Communications were initiated at 18:17, following
the Buchholz alarm being reported, and EDF Energy were requested to remove the
demand from the transformer. At 18:21 EDF Energy called National Grid to confirm
that there was a problem on the transmission system.
16 Such operational communications continued throughout the restoration, with
continuous telephone conversations between control engineers at National Grid and
EDF Energy’s control centre, working together to reconnect the affected area. Some
17 minutes later National Grid offered to restore supplies to Wimbledon and New
Cross.
17 At 18:51 National Grid was called by New Scotland Yard and National Control
informed them that this was a system incident with no third party involvement.
18 The complex and rapidly changing chain of events affected a large number of
organisations. In the wider communication exercise through Thursday night and
Friday, in addition to briefing the media, National Grid was in contact with the
emergency services, the DTI, Ofgem, the London Mayor, energywatch and others.
Page 5National Grid Company plc 10 September 2003
Investigation
19 The planning of maintenance works had been carried out in accordance with National
Grid’s policies and that the maintenance work could not be regarded as a cause of
the incident. The investigation confirmed that the transmission system arrangement
and the communication with the distribution system operator regarding this
maintenance complied with the relevant National Grid planning standards and
operating procedures.
20 All actions in configuring and switching the transmission system complied with
National Grid’s planning standards and operating procedures and that the restoration
process was carried out quickly and professionally without further incident. The
response by control engineers to re-secure the network and restore the balance of
generation and demand ensured that the disturbance was contained within the
affected substations.
21 The reason that the second fault occurred was that an incorrect protection relay was
installed when old equipment was replaced in 2001. This incorrect installation was
not discovered despite extensive quality control and commissioning procedures
followed by both supplier’s and National Grid’s specialist staff. This piece of
equipment has been replaced. Once the cause was known an extensive survey of
similar equipment was immediately initiated. To date 20% (9,000 items) of this type
of equipment on the National Grid system has been surveyed and there have been
no similar cases. The remaining equipment will be surveyed within four weeks.
22 The engineers involved in the commissioning of the automatic protection equipment
had the appropriate training, authorisation, experience and skills to undertake the
task. There is evidence that the detailed commissioning procedures were followed
correctly at all stages and that no part of the process had been omitted. However,
the rating of the automatic protection equipment that is included on the
documentation used for commissioning could have been more clearly visible to the
commissioning engineers.
23 The actions to remove the Hurst transformer did not directly contribute to the cause
of the incident. The consequential increase in flows on the Wimbledon to New Cross
circuit, which were within operational limits, initiated the operation of the protection
relay at Wimbledon. National Grid engineers would not expect their actions to
remove the equipment would have caused the loss of supply.
24 The impact of the incident on the areas of south London was exacerbated by the loss
of supplies to underground and railway transport services.
25 From the 20 July, EDF Energy’s distribution system was arranged such that a
significant supply to London Underground was dependent on a single transmission
circuit. This meant that in the event of a fault occurring on one of National Grid’s
transformers at Wimbledon the distribution system configuration would result in a loss
of supply. However, National Grid understands that EDF Energy had contingency
arrangements for immediate restoration of supplies to London Underground in such
an eventuality.
Page 6National Grid Company plc 10 September 2003
26 Following normal practice, during the incident there was extensive communication
between National Control and the EDF Energy Control Centre, with both control
rooms working effectively together during the incident.
Actions being pursued
27 This is the largest loss of supply from National Grid for over ten years and the
company has expressed its deep regret. This incident involved a number of other
parties and National Grid will be working closely with them in the coming weeks to
examine the consequences and identify improvements in systems or procedures.
National Grid has reviewed its part in the incident and is committed to the following
steps:
· National Grid will work closely with other network operators to identify
any improvements in co-ordination to enhance the overall security of
electricity supplies, particularly to city centres and transport systems.
· National Grid will work closely with EDF Energy, the Mayor, London
Underground, NetworkRail and other London emergency and public
service agencies to establish improved and more responsive
communications in the event of major loss of supply.
· National Grid is urgently surveying all installations as a further check on
the integrity of the automatic protection equipment.
· National Grid will carry out a further comprehensive investigation
examining all aspects of the management of the protection systems so
as to eliminate, as far as possible, the risk of incorrect installation or
operation of automatic protection equipment.
· National Grid will work to review operational procedures, and control
room systems, including alarm presentation, in close consultation with
Ofgem, DTI and other associated parties, to ensure that there is the right
balance between safety risks and supply security.
Page 7National Grid Company plc 10 September 2003
NATIONAL GRID COMPANY PLC
(A wholly owned subsidiary of National Grid Transco plc)
Investigation Report into the Loss of Supply Incident affecting parts of South
London at 18:20 on Thursday, 28 August 2003
This report has been produced by National Grid Company plc (National Grid) to
record the investigation findings concerning the loss of supply in south London on 28
August 2003. The purpose of the investigation is to enable National Grid to identify
the cause or causes of the incident so it may seek to prevent a recurrence. The
purpose of the report is not, however, to identify legal liability; therefore the data and
information within it have not been compiled in accordance with rules of evidence
and cannot be treated as determining either National Grid’s nor any individual’s legal
liability.
Page 8National Grid Company plc 10 September 2003
Investigation Report into the Loss of Supply Incident affecting parts of South
London at 18:20 on Thursday, 28 August 2003
INTRODUCTION
28 National Grid Company plc (National Grid), a wholly owned subsidiary of National
Grid Transco plc, transports electricity and balances the system on a second by
second basis. National Grid delivers electricity from generators and interconnectors
to 12 distribution network operators for local distribution to over 24 million consumers
and directly to a small number of large industrial users. National Grid is the sole
holder of an electricity transmission licence for England and Wales and has a
statutory duty under the Electricity Act 1989 (as amended by the Utilities Act 2000) to
develop and maintain an efficient, coordinated and economical system of electricity
transmission and to facilitate competition in the supply and generation of electricity.
OVERVIEW OF THE INCIDENT
29 On 28 August 2003, two events occurred on the National Grid electricity transmission
system in south London, resulting in an electricity supply failure on the transmission
system from 18.20 until 18.57.
30 The loss of supply occurred following a switching operation to remove a transformer
at Hurst 275kV substation from the transmission system in response to an indication
that a serious alarm had been activated on the transformer or its associated shunt
reactor. Actions were taken to remove the transformer from the system which
required a controlled disconnection of the circuit between Littlebrook and Hurst. For
a short period of 5 to 10 minutes (switching time) this resulted in the supply at New
Cross, Hurst and parts of Wimbledon being dependent on a single transmission
circuit. Within seconds of this operation the circuit between Wimbledon and New
Cross substations automatically disconnected itself. The combination of these two
events was to isolate Hurst, New Cross and a part of Wimbledon 275kV substations
from the main transmission system, disconnecting 724MW of supplies to EDF
Energy’s distribution network.
31 The loss of supply affected 410,000 of EDF Energy’s customers in an area of south
London approximately bounded by Bexley in the east, Kingston in the west, Bankside
in the north and Beckenham in the south, and led to significant disruption to London
Underground and NetworkRail. Supplies from the transmission system to EDF
Energy were restored within 37 minutes.
32 Roger Urwin, National Grid Transco’s Chief Executive Officer initiated a incident
investigation chaired by Nick Winser, Group Director Transmission and Chief
Executive of National Grid Company plc.
33 This is the outcome of the incident investigation into the events of the 28 August
2003.
Page 9National Grid Company plc 10 September 2003
BACKGROUND
The transmission system in south London
34 The National Grid transmission system provides an integrated network for the bulk
transfer of power across England and Wales. The transmission system, which is
operated at 400,000 volts and 275,000 volts, connects major power stations and
delivers electricity to the regional distribution networks. The peak demand on the
England and Wales transmission system is around 54,400MW. The demand for
electricity in the Greater London area represents about 20% of the total transmission
system demand in England and Wales.
35 The transmission system has been designed and built for an expected life of between
15 years and 80 years, depending on the type of asset.
36 There are no large generation stations connected directly to the transmission system
in the Central London area, although large power stations exist close to London at
Barking, Grain, Littlebrook and Kingsnorth. The transmission system facilitates the
transmission of power from these and more remote generating stations to London.
Figure 2: Transmission system in south London
© Crown Copyright, National Grid Transco EL273384
37 The south London 275kV network between Wimbledon and Littlebrook is shown in
the above figure. The network is made up of substations, which include switchgear,
transformers, shunt reactors and protection and control equipment. These are
connected by circuits comprising overhead lines and cables. A description of the
assets that make up the south London network is provided in appendix 1.
Page 10National Grid Company plc 10 September 2003
Investment Programme in the Area
38 Since 1990, around £3,600m has been invested in the National Grid transmission
system. Of this, approximately £700m has been invested in the transmission system
in the Greater London area.
39 Major elements of this work include connection works for new generation at Barking
and Kingsnorth, construction of two new substations at West Ham and St Johns
Wood, and major infrastructure reinforcement including the new 20km cable between
Elstree and St Johns Wood.
40 Since 1995/96 investment in the London area has been increasing and has been
around £100m per year for the last 2 years as the new Elstree-St Johns Wood tunnel
and cable circuit has been constructed.
Figure 3: Investment in the London area
London Area Investment
120
100
80
£m Outturn
60
40
20
0
95/96 96/97 97/98 98/99 99/00 00/01 01/02 02/03
41 Of the investment in the London area, around £75m has been invested in the
Littlebrook to Wimbledon 275kV system and the adjacent network in south London.
Specific projects have included:
· Supply point reinforcement works at Littlebrook and New Cross
· 275kV cable works on the circuits between between Hurst, New Cross and
Wimbledon over the period 1995 to 2002
· Works to provide a new tunnel under the River Thames at Dartford and new
275kV cables for the Littlebrook to West Thurrock 275kV circuits
· Switchgear replacement
· Automatic protection and control system replacement.
· Environmental improvement works at several sites, such as enhancing oil
containment works.
42 As part of National Grid’s planned asset replacement programme, future work in the
Littlebrook to Wimbledon 275kV system and the adjacent network in south London
Page 11National Grid Company plc 10 September 2003
includes the replacement of shunt reactors, replacement of the Beddington-Rowdown
275kV cable, the extension of the supply point at New Cross and improvements to
cable cooling systems and joint bays.
43 The average expenditure in the London area is planned to be over £50m per year
over the next 5 years.
44 There has been a considerable investment programme in the transmission
system in and around London since 1990, and this programme is set to
continue at a high level in future years.
Key Policies and Procedures Relevant to the Incident
45 As part of National Grid’s responsibility to operate a safe, secure and reliable
transmission system it has a responsibility to ensure that the asset related safety,
environmental and operational risks are managed and acceptable. In addition, the
company has an obligation to carry out this duty in an efficient manner. To this end
National Grid has developed an asset management approach which uses a
combination of maintenance, refurbishment and replacement strategies. The
procedures, which provide a framework for the delivery of this approach, are well
established and defined and subject to external audit as part of National Grid’s ISO
9001 accreditation. Details on National Grid’s maintenance, asset replacement and
commissioning policy are included in appendix 2.
Page 12National Grid Company plc 10 September 2003
FINDINGS
Operating Arrangements on the day
46 On the evening of Thursday 28 August 2003, the transmission system in the south of
London was secure and was operating in accordance with the relevant National Grid
planning standards and operating procedures (Appendix 3 provides details). The
substations were all configured in a secure manner supplying normal demands of
around 1,100MW.
47 The transmission system in the area was arranged with a number of circuits out of
service for scheduled maintenance.
Circuit Reason Dates
Installation of thermal monitoring on cables,
Number two circuit from 26 August to
maintenance of circuit breakers and other
Littlebrook to Hurst 19 September
planned maintenance at Hurst and Littlebrook
Major refurbishment of the cables and
Number one circuit from 1 July to
installation of new protection and control
Wimbledon to New Cross 28 September
systems
48 The Wimbledon, New Cross and Hurst substations, that were to be affected by the
incident, were connected to the rest of the transmission system by two circuits,
ensuring that a single transmission fault would not result in a loss of supply.
49 A simple diagram of the transmission network in this area of London is illustrated in
the figure below.
Figure 4: Schematic of the Transmission System in South London
Northfleet
Willesden 1 1 1 West
Beddington Wimbledon New Cross Hurst Littlebrook
Beddington 2 2 2 Beddington
& Kemsley
Out of service for
scheduled maintenance
50 If a fault did occur and left supplies dependent on a single transmission circuit, in
most cases, National Grid was able to restore security of supply within switching time
or by returning one of the circuits that was out on maintenance.
51 To maintain an efficient transmission system it is necessary to undertake planned
maintenance. The disconnection of circuits and network configuration is planned and
agreed following a rigorous process that involves studies to both optimise the
coordination of outages with parties connected to the system and to ensure the
network configuration is secure against all credible faults that may occur.
52 The planning process to agree these outages and the configuration of the system
began in July 2002. As part of the process regular liaison meetings were held and
exchanges of information undertaken with EDF Energy (on a daily basis in the last
two months). These discussions concluded in a formal agreement for the release of
Page 13National Grid Company plc 10 September 2003
the circuits before the work was due to commence. As is normal, the maintenance
schedules of both EDF Energy and National Grid were subject to change during the
early part of the summer. EDF Energy were fully engaged in this process and fully
aware of the configuration of the transmission system and its impact on its system.
As part of this process the contingency plans for EDF Energy’s 132kV Wimbledon
substation were discussed, as the planned outage would entail a reduction in the
number of feeds from the National Grid transmission system from four to three.
National Grid understood that due to limitations on the EDF Energy’s system, some
distribution supplies from Wimbledon would be dependent on a single transmission
circuit. National Grid understands that in the event of the loss of this transformer
EDF Energy’s post fault action would be to immediately switch its Wimbledon Grid
132kV substation to reinstate supplies, from the remaining two National Grid
transformers.
53 If the circuits that were out for maintenance had been available, clearly there would
have been no loss of supply. However maintenance is an essential part of sustaining
an efficient transmission system and to increase security above the current standards
would require a huge investment in new transmission assets.
54 The investigation confirmed that the configuration of the transmission system
was not a contributory factor to the loss of supply.
Sequence of Events
55 The sequence of events commenced on Thursday 28 August at 18:11 when an alarm
was received at the Electricity National Control Centre (National Control) at
Wokingham. The system configuration at the start of the incident and the power
flows are illustrated below.
Figure 5: Transmission system configuration and power flows
486 MW
Northfleet
Willesden 1 1 1 West
Beddington Wimbledon New Cross Hurst Littlebrook
Beddington 2 2 2 Beddington
72 MW & Kemsley
Supplying EDF Supplying EDF Supplying EDF Supplying EDF
478 MW 359 MW 199 MW 84 MW
Out of service for
scheduled maintenance
56 The sequence of events during the incident is described below. The detailed switch
operations is attached in appendix 4.
57 At 18:11 staff at the National Control received an indication that a transformer or
shunt reactor at Hurst substation was in distress and could fail, with potentially
significant safety and environmental impacts. The indication, called a “Buchholz
alarm”, told National Control that gas had accumulated within the oil inside the
transformer or shunt reactor, which can lead to equipment failure.
Page 14National Grid Company plc 10 September 2003
58 At 18:17 discussions were held with EDF Energy regarding the Hurst transformer or
shunt reactor. National Control informed EDF Energy that the transformer was to be
switched out of service. To achieve this the transmission system had to be
rearranged by switching equipment and circuits in and out, so the affected equipment
could be safely and securely taken out of service. During switching time (typically 5 –
10 minutes) one circuit would supply New Cross and Hurst substations.
59 EDF Energy confirmed that they had disconnected the transformer from the
distribution system. There was no impact on supplies to EDF Energy as these
remained connected to Hurst substation via two other transformers.
60 The immediate priority was security of supplies. At 18:19 the circuit from Littlebrook
to Beddington and Kemsley was switched in. This reconfiguration of the network
ensured power flows at Littlebrook substation would be secure once the number one
circuit from Hurst to Littlebrook was switched to take the Hurst transformer and
associated Hurst shunt reactor three out of service.
61 At 18:20 two circuit breakers were opened at Hurst to remove the transformer or
shunt reactor from service. At this point, the Hurst and New Cross substations were
supplied from Wimbledon 275kV substation and were dependent on the single,
number two circuit, from Wimbledon to New Cross.
Figure 6: Transmission system at 18.20
Northfleet
Willesden 1 1 1 West
Beddington Wimbledon New Cross Hurst Littlebrook
Beddington 2 2 2 Beddington
558 MW & Kemsley
Supplying EDF Supplying EDF Supplying EDF Supplying EDF
478 MW 359 MW 199 MW 84 MW
Out of service for
National Control action
scheduled maintenance
62 Immediately following the opening of the two circuit breakers at Hurst, the automatic
protection relay operated on the number two circuit from Wimbledon to New Cross
automatically opening two circuit breakers at Wimbledon and removing the number
two circuit from Wimbledon to New Cross from service. This isolated New Cross,
Hurst and part of Wimbledon substations from the rest of the system. All supplies
were lost at Hurst and New Cross substations and 35% of the supplies were also lost
to EDF Energy’s Wimbledon Grid 132kV substation. Two transformers at Wimbledon
continued to supply Wimbledon Grid 132kV substation.
63 The transmission network was now configured as in the diagram below.
Page 15National Grid Company plc 10 September 2003
Figure 7: Transmission system immediately following the incident
Northfleet
Willesden 1 1 1 West
Beddington Wimbledon New Cross Hurst Littlebrook
Beddington 2 2 2 Beddington
& Kemsley
Supplying EDF Supplying EDF Supplying EDF Supplying EDF
166 MW lost 359 MW lost 199 MW lost 84 MW maintained
312 MW maintained
Out of service for
National Control action Automatic action
scheduled maintenance
64 Following the event EDF Energy transferred 72MW of demand supplied by
Wimbledon Grid 132kV substation to another supply point by reconfiguring the
distribution network.
65 Assessing the alarms received, National Control concluded that the automatic
protection equipment on the number two circuit from Wimbledon to New Cross had
most likely operated incorrectly.
66 At 18:21 National Control and EDF Energy discussed the loss of supply and the
substations affected.
67 At 18:22 standby engineers were called out to Wimbledon, New Cross and Hurst
substations to investigate and help restore supplies.
68 At 18:23 National Control commenced the sequence of operations to restore security
for the remaining transmission system and restore supplies. It was decided to keep
the number two circuit from Wimbledon to New Cross out of service until the cause of
the operation of the automatic protection relay was established, as it was highly
probable that if switched into service the automatic disconnection would recur.
69 At 18:25 the network was reconfigured to isolate the number two circuit from
Wimbledon to New Cross, while Wimbledon substation was fully energised by closing
the two circuit breakers which had automatically opened following the earlier
operation of the automatic protection equipment. These actions re-secured the
transmission system against further faults, minimising the chance of further losses of
supply. Due to uncertainty over the cause of the protection operation, the re-
energised transformer feeding Wimbledon Grid 132kV substation was not
immediately made available to EDF Energy, although two transformers at Wimbledon
capable of carrying the entire demand remained in service throughout the incident.
70 Having re-secured the system at 18:25 the restoration strategy was to configure the
network for a phased re-energisation starting at Littlebrook. A complex switching
sequence was required to prepare the transmission and distribution network and
ensure that the risk of further faults was minimised by carefully switching cable
circuits to control the voltage.
71 By 18:30 further reconfiguration of the network had taken place and the first sections
of Hurst and New Cross substations had been re-energised.
Page 16National Grid Company plc 10 September 2003
72 At 18:31 National Control confirmed those developments to EDF Energy Control and
informed them that it would contact them soon to start restoring supplies.
73 At 18:38 National Control informed EDF Energy Control that supplies could be
restored to Wimbledon Grid 132kV substation. During the same conversation
National Control also informed EDF Energy Control that supplies at New Cross could
be restored. At this time EDF Energy Control requested time to assess the
distribution network and agreed to call back.
74 By 18:40 further reconfiguration of the network had taken place energising further
sections at New Cross and Hurst.
75 At 18:40 National Control contacted EDF Energy Control to offer restoration of
supplies at Hurst. EDF Energy reconfigured the distribution network and EDF
Energy’s Bromley supplies were restored at 18:44. This restored approximately a
third of the 410,000 customers lost at 18:20.
76 Between 18:44 and 18:50 further reconfiguration of the transmission system took
place.
77 At 18:48 EDF Energy Control contacted National Control to complete restoration of
supplies to the Wimbledon Grid 132kV substation. At 18:52 all remaining supplies to
Wimbledon Grid 132kV substation were restored.
78 At 18:51 a further Buchholz alarm was received relating to the transformer or shunt
reactor at Hurst. No indications were received that the transformer had been
disconnected by automatic protection, indicating that the shunt reactor was the faulty
equipment.
79 At 18:51 New Scotland Yard contacted the National Grid control room and it was
confirmed that the loss of supply was a system incident, with no third party
involvement.
80 At 18:52 National Control contacted EDF Energy Control to restore supplies at New
Cross. EDF Energy Control requested time to assess the distribution network prior to
restoring supplies and agreed to call back.
81 At this stage 29 switching operations had been planned and successfully executed in
26 minutes.
82 At 18:56 EDF Energy Control called National Control back and EDF Energy supplies
to New Cross were restored at 18:57. At this point all supplies from the transmission
system were available to the distribution network.
83 At 19:10 EDF Energy Control contacted National Control and asked for the Hurst
transformer to be returned to increase security on the distribution network. The
original Buchholz alarm was attributable to the shunt reactor at Hurst which was
isolated and the transformer was made available to EDF Energy Control.
Page 17National Grid Company plc 10 September 2003
84 At 19:14 EDF Energy Control confirmed all supplies to consumers had been
restored.
85 During the period 19.00 to 19.45 standby site engineers arrived at the three sites. On
arrival each standby engineer checked protection relay indications and alarm logs
and confirmed the situation with National Control. To support the restoration process
and initial investigation, two further engineers were also called to attend site.
86 The network was now configured as follows:
Figure 8: Transmission system after restoration
437 MW
Northfleet
Willesden 1 1 1 West
Beddington Wimbledon New Cross Hurst Littlebrook
Beddington 2 2 2 Beddington
& Kemsley
Supplying EDF Supplying EDF Supplying EDF Supplying EDF
422 MW 262 MW 175 MW 84 MW
Out of service for
National Control action Automatic action
scheduled maintenance
87 At 20:02 restoration of all supplies was confirmed with New Scotland Yard.
88 At 20:55 EDF Energy Control requested a formal report on the incident.
89 After assessment by engineers on site at Wimbledon, it was confirmed the automatic
protection relay had been taken out of service and the number two Wimbledon to
New Cross circuit was returned to service at 23:00. This action further enhanced
security at Wimbledon and Hurst. No further switching was carried out to allow the
system to be re-assessed and minimise the risk of any further faults. Full security
was completed by reconfiguring the network overnight, with full security re-
established at 01:05 on 29 August.
90 The Standby engineers called to site remained on each site until restoration was
complete.
91 Following the initial loss of supply National Control engineers correctly
assessed the risks and restored transmission supplies within 37 minutes.
Communication during the incident
92 Throughout the incident close communication was maintained between National
Control and EDF Energy. Communications relating to this incident commenced at
18:17 following receipt of the initial alarm and were maintained throughout the
incident and well into the night. In addition, National Control received a call from New
Scotland Yard, and confirmed that this was a system incident.
Page 18National Grid Company plc 10 September 2003
93 The incident generated substantial media interest, with National Grid handling some
300 media calls on Thursday evening.
94 A verbal statement was given to journalists within 25 minutes of the incident. Written
media statements were issued at 21.25 on Thursday evening and 13.30 on Friday
afternoon.
95 During a major incident, National Grid would normally communicate with bodies such
as Ofgem, DTI, energywatch and others with a direct interest, depending on the
incident. Contact was made with Ofgem, DTI and other parties as soon as possible
during the incident and communication continued through the evening and the
following days.
96 Following the event there were high-level contacts with Ofgem, the Energy Minister,
DTI officials, the Mayor of London, EDF Energy and energywatch, among others.
Response to the Buchholz Alarm
97 National Control actions were in line with National Grid procedures for responding to
an indication that a Buchholz alarm had been activated on a transformer or shunt
reactor at Hurst 275kV substation.
98 The causes and implications of a Buchholz alarm are set out in appendix 5, but in
summary, the alarm provides a warning of potential problems in the transformer or its
associated shunt reactor that could result in a major failure. Hence, due to the nature
of the consequences of such a failure, National Grid procedures specify the
equipment is to be disconnected from the transmission system, except in a limited
number of circumstances. These exceptions include any action that would result in a
loss of supply.
99 As is normal in the design of control room systems, to avoid “alarm flooding” in the
event of major system incidents, alarms are combined to reduce the total number
displayed in the control room. The investigation noted that the grouping and
nomenclature of the alarms for the transformer and shunt reactor did not clearly
indicate whether the transformer or the associated shunt reactor was the origin of the
gas alarm.
100 When the alarm was received, National Control took immediate action to begin the
process to remove the transformer by asking EDF Energy to disconnect it from the
distribution system.
Page 19National Grid Company plc 10 September 2003
Disconnection of the Transformer
101 National Control undertook further switching actions in order to disconnect the
transformer from the transmission system. The specific design of the substation,
called a “mesh”, required the disconnection of the circuit from Littlebrook to Hurst.
The switching plan undertaken was in accordance with National Grid procedures on
operating mesh substations which, for the five to ten minutes taken to switch, left the
electricity supply at New Cross and Hurst substations dependent on a single circuit.
102 The disconnection of the Littlebrook to Hurst circuit re-routed power and as expected
increased the power flows on the number two circuit from Wimbledon to New Cross
from 72MW (213MVA) to 558MW (695 MVA). This is comfortably within the design
ratings of 815 MVA for the circuit.
103 The investigation confirmed that the operational decision to switch out the Littlebrook
to Hurst circuit and supply Hurst and New Cross from a single circuit from
Wimbledon, for the five to ten minutes required to complete the switching, was in
accordance with operating procedures and took account of the need to remove the
safety risk of a major failure of a transformer.
104 The investigation has also confirmed that the configuration and capability of the
system was in accordance with National Grid’s standards and procedures, and that
National Grid undertakes an average of 2,700 annual switching operations at mesh
corners without incident.
105 The investigation has found that National Grid engineers would not expect
their actions in removing the equipment to have caused a loss of supply.
Unexpected Operation of the Protection
106 At National Grid’s Wimbledon substation, the automatic protection equipment
associated with the number two circuit from Wimbledon to New Cross, detected the
change in power flows as a result of the switching at Hurst, as a fault. The protection
equipment disconnected the circuit to prevent damage to other equipment and/or the
propagation of the fault through the transmission system.
107 In this case the protection relay that operated was being used for backup protection.
Backup protection is fitted to the transmission network, in conjunction with the main
protection and is designed to disconnect faults not cleared by the main protection
equipment.
108 The protection equipment that operated was an Inverse Definite Minimum Time
(IDMT) relay, a commonly used type. It does not operate immediately, but starts to
operate when the electric current on the circuit exceeds a certain threshold. The
speed of operation depends on how far the measured current is above the threshold
level.
109 The protection relay had been correctly specified during the design process and the
settings sheet had been correctly produced. However the relay that had been
physically supplied and installed at Wimbledon was a 1 ampere rated relay, not the 5
Page 20National Grid Company plc 10 September 2003
ampere relay specified on the settings sheet. In all other respects the settings on the
relay were correct and were confirmed during several check points in the
construction and commissioning process.
110 The effect of installing a 1 ampere relay instead of a 5 ampere, meant that the
current flow at which the protection would operate was five times lower than the
correct rating and below the rating of the circuit itself.
111 The 1 ampere protection relay was set to operate at a current of 1,020 amperes on
the transmission circuit and was triggered on the day by a current of 1,460 amperes.
This is significantly below the operating capability of the cable, at 4,450 amperes and
the original specification of the protection relay, at 5,100 amperes (see appendix 6).
112 The protection relay was commissioned in June 2001 as part of a replacement
scheme. Following a survey conducted as a result of the incident, all the automatic
protection equipment in the area was surveyed and found to be correctly installed. A
full survey of similar equipment at all substations in England and Wales has been
initiated, and to date, having completed 20% of the total, no further cases have been
revealed.
113 The incident investigation found that despite rigorous processes for commissioning
protection equipment, the wrong protection relay was installed and commissioned at
Wimbledon substation and this caused the number two circuit from Wimbledon to
New Cross to automatically disconnect unexpectedly, and caused the loss of supply.
114 The commissioning of the automatic protection equipment involved a number of
stages as set out in appendix 2. The investigation has found evidence to support
that the relay settings had been correctly calculated. The setting sheets were
correctly produced and signed by both the engineer who calculated the settings and
the engineer who confirmed the application of those settings to the protection
equipment. Furthermore the investigation confirmed the protection equipment had
been tested by the manufacturer in accordance with industry practice, and that pre-
energisation inspection tests were carried out. There is evidence that the rating of
the automatic protection equipment that is included on the documentation used for
commissioning could have been more clearly set out and hence visible to the
commissioning engineers. The investigation found no evidence that any part of the
commissioning process had been omitted.
115 The investigation has found that the direct cause of the loss of supply was the
incorrect operation of a backup protection relay on the number two circuit from
Wimbledon to New Cross.
Maintenance of Assets
116 National Grid has an established maintenance policy and the assets involved in the
incident have all been maintained according to that policy.
117 The following table summarises the maintenance undertaken on the assets involved
in the incident at Wimbledon and Hurst.
Page 21National Grid Company plc 10 September 2003
Item Description Last Maintenance Next scheduled
undertaken maintenance
Number two circuit from
Wimbledon to New Cross, Commissioned 2001 2007
backup protection
Hurst Transformer 3 September 02 2006
Hurst Transformer 3 Shunt
March 03 2009
Reactor
118 The investigation found that an appropriate level of maintenance had been
carried out on the assets and poor asset condition was not a contributing
factor to this incident.
Other Factors
119 The investigation has determined that together with the above factors that are directly
attributable to the operation of the transmission system, there were a number of other
factors, external to the transmission system, that may have contributed to the
duration or scale of the incident.
Configuration of EDF Energy’s Wimbledon Grid 132kV Substation
120 EDF Energy own and operate the 132kV substation at Wimbledon, which is
physically located on different site to National Grid’s substation. Maintenance
outages were agreed between National Grid and EDF Energy as part of a well
defined process and significant information was exchanged on network configuration
and contingency plans for faults.
121 Four transformers from National Grid’s 275kV substation at Wimbledon supply EDF
Energy’s 132kV substation. Normally all four transformers are connected ensuring
that supplies can be maintained for the loss of any one transformer. National Grid
understands that EDF Energy splits its Wimbledon substation into two parts to
reduce fault currents and prevent over-stressing the equipment. Normally, with two
transformers supplying each part.
122 When National Grid requires one of the transformers to be taken out of service for
maintenance, EDF Energy configures its network with one transformer on one part
and two on the second (figure 9). Two transformers supply the majority of demand
for Wimbledon and Wandsworth. The remaining transformer supplies the remaining
demand at Wimbledon and Wandsworth, together with demand at Lots Road for
London Underground.
123 If one transformer is out of service for maintenance the Lots Road circuits will always
be dependent on a single transmission circuit, because they can only be connected
to the single transformer.
Page 22National Grid Company plc 10 September 2003
Figure 9: Configuration of EDF Energy's distribution system
National Grid EDF Energy London
Underground
Wimbledon
275kV
Lots Road
Wimbledon
Grid 132kV
Out of Service for scheduled maintenance
124 The configuration of Wimbledon Grid 132kV substation on 28 August 2003 is
illustrated in the diagram above. National Grid maintained supplies to the section
connected to transformers two and four throughout the incident.
125 National Grid understands that when one of its transformers is out of service and in
the event of a fault on the remaining transformer, the normal arrangement would be
for EDF Energy to connect the two parts of the Wimbledon Grid 132kV substation.
However, National Grid does not know whether, in these particular circumstances,
EDF Energy would have been able to take such post-fault action.
126 The investigation found that the configuration of the EDF Energy’s distribution
system was not a contributory factor to the initiation of the incident. However,
a more rapid implementation of post-fault actions or an alternative
configuration could have mitigated the overall impact of the incident, reducing
the duration and perhaps the scale of the loss of supply.
Communications
127 Following normal practice, during the incident there was extensive communication
between National Control and the EDF Energy Control Centre. Communications
were initiated at 18:17, when the initial Buchholz alarm was reported, and EDF
Energy were requested to remove the demand from the transformer. At 18:21 EDF
Energy called National Control to confirm that there was a problem on the network,
and 17 minutes later National Control called back offering to restore supplies to
Wimbledon and New Cross.
128 Such operational communications continued throughout the restoration, with
numerous telephone conversations between National Control engineers and EDF
Energy’s control engineer, working together to reconnect the affected area.
129 Following the restoration of supply, communications with the control rooms continued
as further reconfiguration of the systems took place to ensure full security was
restored.
130 During the incident National Control managers were confident that this was a system
incident and this was confirmed to New Scotland Yard at 18.51.
Page 23National Grid Company plc 10 September 2003
131 From Thursday evening and over the next day, National Grid gave working level and
senior level briefings to DTI (including the Energy Minister), Ofgem, the Mayor of
London, energywatch and many others with a direct interest.
132 National Grid began responding to the very large number of media calls within 25
minutes of the start of the incident through its communication procedures. Senior
executives from the company were available for media interview between late
Thursday evening and early Saturday morning.
133 The investigation has found a crucial factor in communications during the incident
was that, although National Grid was able to restore supplies to its network within 30
minutes, the various services to the public returned to normal in different timescales
and in different ways. For example, after re-configuring the distribution network EDF
Energy was able to restore supplies to some of its customers before supplies were
restored by National Grid, but some of its customers could only be restored a short
time afterwards. Disruption to rail services continued after power was restored due to
timetables being disrupted and the evacuation of trains.
134 The prime route for communications with the public is generally through the standard
channels of the providers of these services. For instance it would be through the
customer call centre of EDF Energy and through the railway companies’ passenger
information units. This was appropriate, as only these service providers could let the
public know how the incident had affected their operations.
135 The providers of key services to the public, such as the underground and railway
network operators, typically draw their electricity supplies from the local distribution
network. During an incident involving loss of electricity supplies, including on the
National Grid system, that would typically expect to communicate with the distribution
company.
136 The investigation has found that further work is required as to whether
enhanced communication between National Grid and the various organisations
providing key services to the public during such a major incident would help
them in making decisions on how to respond to the incident and communicate
about their services with the public.
Investigation
137 The planning of maintenance works had been carried out in accordance with National
Grid’s policies and that the maintenance work could not be regarded as a cause of
the incident. The investigation confirmed that the transmission system arrangement
and the communication with the distribution system operator regarding this
maintenance complied with the relevant National Grid planning standards and
operating procedures.
138 All actions in configuring and switching the transmission system complied with
National Grid’s planning standards and operating procedures and that the restoration
process was carried out quickly and professionally without further incident. The
response by control engineers to re-secure the network and restore the balance of
Page 24National Grid Company plc 10 September 2003
generation and demand ensured that the disturbance was contained within the
affected substations.
139 The reason that the second fault occurred was that an incorrect protection relay was
installed when old equipment was replaced in 2001. This incorrect installation was
not discovered despite extensive quality control and commissioning procedures
followed by both supplier’s and National Grid’s specialist staff. This piece of
equipment has been replaced. Once the cause was known an extensive survey of
similar equipment was immediately initiated. To date 20% (9,000 items) of this type
of equipment on the National Grid system has been surveyed and there have been
no similar cases. The remaining equipment will be surveyed within four weeks.
140 The engineers involved in the commissioning of the automatic protection equipment
had the appropriate training, authorisation, experience and skills to undertake the
task. There is evidence that the detailed commissioning procedures were followed
correctly at all stages and that no part of the process had been omitted. However,
the rating of the automatic protection equipment that is included on the
documentation used for commissioning could have been more clearly visible to the
commissioning engineers.
141 The actions to remove the Hurst transformer did not directly contribute to the cause
of the incident. The consequential increase in flows on the Wimbledon to New Cross
circuit, which were within operational limits, initiated the operation of the protection
relay at Wimbledon. National Grid engineers would not expect their actions to
remove the equipment would have caused the loss of supply.
142 The impact of the incident on the areas of south London was exacerbated by the loss
of supplies to underground and railway transport services.
143 From the 20 July, EDF Energy’s distribution system was arranged such that a
significant supply to London Underground was dependent on a single transmission
circuit. This meant that in the event of a fault occurring on one of National Grid’s
transformers at Wimbledon the distribution system configuration would result in a loss
of supply. However, National Grid understands that EDF Energy had contingency
arrangements for immediate restoration of supplies to London Underground in such
an eventuality.
144 Following normal practice, during the incident there was extensive communication
between National Control and the EDF Energy Control Centre, with both control
rooms working effectively together during the incident.
Page 25National Grid Company plc 10 September 2003
Actions being pursued
145 This is the largest loss of supply from National Grid for over ten years and the
company has expressed its deep regret. This incident involved a number of other
parties and National Grid will be working closely with them in the coming weeks to
examine the consequences and identify improvements in systems or procedures.
National Grid has reviewed its part in the incident and is committed to the following
steps:
· National Grid will work closely with other network operators to identify
any improvements in co-ordination to enhance the overall security of
electricity supplies, particularly to city centres and transport systems.
· National Grid will work closely with EDF Energy, the Mayor, London
Underground, NetworkRail and other London emergency and public
service agencies to establish improved and more responsive
communications in the event of major loss of supply.
· National Grid is urgently surveying all installations as a further check on
the integrity of the automatic protection equipment.
· National Grid will carry out a further comprehensive investigation
examining all aspects of the management of the protection systems so
as to eliminate, as far as possible, the risk of incorrect installation or
operation of automatic protection equipment.
· National Grid will work to review operational procedures, and control
room systems, including alarm presentation, in close consultation with
Ofgem, DTI and other associated parties, to ensure that there is the right
balance between safety risks and supply security.
Page 26You can also read
